This invention relates to electron multipliers and more particularly to electron multipliers of the channel plate type. The invention is applicable to channel plates for use in electronic imaging tube applications.
Herein, a channel plate is defined as a secondary-emissive electron-multiplier device comprising a stack of conducting sheet dynodes, insulated from one another, and having a large number of channels passing transversely through the stack. Each channel comprises aligned holes in the dynodes, and the walls of the holes are capable of secondary electron emission. In use, the dynodes are held at progressively increasing positive d.c. voltages from input to output. Electrons incident upon the wall of the hole of the input dynode of a channel give rise to an increased number of secondary electrons which pass down the channel to fall upon the wall of the hole of the next more positive dynode where further secondary emission multiplication occurs. This process is repeated down the length of each channel to give a greatly enhanced output electron current substantially proportional to the input current. Such channel plates and methods for manufacturing them are described in U.K. Patent Specification No. 1,434,053.
Channel plates may be used for intensification of electron images supplied either by the raster scan of the electron beam of a cathode ray tube or by a photocathode receiving a radiant image which excites photoelectrons which are fed as a corresponding electron image to the input face of the channel plate. In either event, electrons fall on the portions of the input face of the first dynode of the channel plate between the channels, exciting secondary electrons which, by reason of their spread in emission energy and direction, pursue trajectories in the space in front of the channel plate which carry them into channels remote from their point of origin. The contrast and definition of the image are degraded by each channel receiving additional input electrons in proportion to the original input electron density at channels over a range of distances away.
The sheet dynodes may be made from a metal alloy such as aluminum magnesium or copper beryllium which is subsequently activated by heating in an oxygen atmosphere to produce a surface all over the dynode which has a high secondary emission coefficient. The input face will thus have an undesirably high secondary emission leading to contrast degradation. Alternatively, the dynodes may be made from sheet steel coated with cryolite, for example, to give a secondary emission coefficient of 4 or 5. In this case it is also impractical to restrict the coating of cryolite to the insides of the holes and the input face will again have an undesirably high secondary emission coefficient.
Moving the channels closer together to minimize the flat surface between adjacent holes on the input face is unsatisfactory for a number of reasons. Firstly, the ratio of hole area to metal area is increased and the individual dynodes become flimsy and difficult to handle during plate manufacture. Secondly, since the most readily made channels have a circular cross-section, the flat area between channels could not be eliminated, even with the closest channel spacing. Finally, an important application of channel plate multipliers is to color display devices in which color selection takes place at the multiplier output. For example, a pair of selector electrodes may be provided on the output face of the stack, each electrode consisting of regularly spaced strips of conductor, the strips being in registration with lines of channels and lines of phosphor on the screen. The strips of the two selector electrodes are interdigitated and voltages are applied to the electrodes to deflect each of the channel output beams onto a selected phosphor. Such a color selection system is described in U.K. Pat. No. 1,458,909. Close channel spacing leaves less space for color selection electrodes and also less space on the screen for the corresponding pattern of phosphor stripes or dots.